Experimental and simulation study on CO₂ mineralization under diffusion control

Because basalt formation is rich in calcium, magnesium, iron and other bivalent metal elements, it can transform injected CO₂ into carbonate rock through efficient mineralization reaction, so as to achieve a safe and effective permanent storage of CO₂. However, the research on the mechanism of CO₂ mineralization and storage is incomplete, and most studies ignore the effect of diffusion on the mechanism of CO₂ mineralization. By conducting experiments with the sand grains being filled in the tube or scatted in a beaker, the CO₂-water-rock reactions with different minerals, different particle sizes and different reaction time are investigated. The rock samples after the reaction are analyzed by multi-dimensional analysis, including Raman spectra, inorganic carbon content and SEM-EDS, thus clarifying the importance of diffusion in the CO₂ mineralization and the spatio-temporal evolution of CO₂ mineralization products under diffusion control. At the same time, the numerical simulation model of the tube filling experiment is established by TOUGHREACT, and the accuracy of the numerical simulation model is ensured by fitting the physical experiment results. On this basis, more studies on the law of the influencing factors are carried out, and the mechanistic explanation for the physical experimental phenomena is provided. The results of physical experiment and numerical simulation show that: ① For the experiment carried out with olivine grains scattered at the bottom of beaker, no diffusion occurred. At this situation, after 14 days’ reaction, no mineral carbonation was observed. Meanwhile, after 28 days marginal amount of magnesite was observed. ② In contrast, in the filled tubes, under the control of diffusion, the main precipitates in the olivine filled tube are magnesite, and the precipitates in the natural basalt filled tube are mainly calcite and siderite, with magnesite as auxiliary. At the same mineral particle size and the same reaction time, the CO₂ mineralization rate in natural basalt is much smaller than that of olivine, because olivine has the fastest dissolution rate compared with other constituent minerals of basalt. ③ In olivine or basalt filled tubes, the carbonate precipitates are distributed ununiformly along the filled tubes. The root cause of this phenomenon is explained by numerical simulation, i.e., under the control of diffusion, there are concentration gradients of pH, DIC and bivalent metal cations in the packed bed, and the concentration gradients are different in different directions. ④ The specific mineral surface has a significant influence on the CO₂-water-rock reaction, which not only affects the mineralization efficiency but also affects the spatial distribution of the mineralized products, and finally affects the porosity distribution of the porous media after the reaction. ⑤ Compared with pressure, temperature has more influence on CO₂ mineralization. The precipitation of calcite, siderite and carbonate increases significantly when the temperature increases from 65 ℃ to 85 ℃, but only the precipitation of magnesite increases significantly when the temperature increases from 85 ℃ to 100 ℃. The pressure has little effect on the formation of calcite precipitate, but has no effect on the formation of magnesite and siderite.